Pittsburgh's nickname is "the city of bridges." It purportedly has three more than Venice.

Of course, thought kind of had to be put into it. The city’s history of steel production—at one time 1/3 to 1/2 of the country’s total—led to smog that rivaled the worst of the Industrial Revolution. Through an extensive cleanup campaign, air pollution has been steadily decreasing for years, but in 2011 the American Lung Association still ranked it the 3rd worst city in the U.S. for short-term particle pollution. Still, the transformation is as impressive as Boston Harbor’s. Check out Pittsburgh Green Story for more info.